1. Field of Invention
This invention relates to optical fibers and waveguides, specifically to writing gratings in fibers and waveguides.
2. Prior Art
Optical fibers are now widely used for carrying light signals in optical communication systems. Alterations in the refractive index in the fibers (known as xe2x80x9cgratingsxe2x80x9d) are used to control those light signals. Fiber gratings can serve as filters, routers, modulators, and attenuators, can be used to process different channels in a wavelength-division multiplexed (WDM) telecommunication system, or can be used to control the output of lasers.
Several methods have been proposed for fabricating fiber gratings. A fiber can be illuminated from the side with a periodic pattern of ultraviolet light, causing a permanent change in the refractive index of the fiber""s photosensitive core. The resulting periodic index pattern forms the fiber grating.
The time required to write a grating depends on the photosensitivity of the fiber""s glass core. Photosensitivity is essentially the inverse of how much laser fluence is needed to obtain fixed grating strength. For fibers with high concentration ( greater than 10% mol.) of germanium in their core, photosensitivity is high and the time required to write a grating is minimized. However for some applications the numerical aperture of such fibers can be undesirably high (numerical aperture, NA greater than 0.2). For standard communication fibers (NAxcx9c0.12) having less than 3% mol. germanium in the core, the photosensitivity of the core is small. Writing a grating in such fibers requires a considerable fluence of ultraviolet light, thus increasing the time to write the grating. For Bragg gratings, the period of the grating is typically xcx9c0.5 micron, so even very slight movements or drift in the position of the fiber during writing can ruin the grating.
It is known that immersing an optical fiber in an atmosphere of high-pressure hydrogen for many hours can increase the photosensitivity of the fiber, but this is a tedious and possible dangerous procedure. Alternatively, the fiber can be doped with a high concentration of Ge in the core and then co-doped with boron to reduce the NA of the fiber. Such fibers have an increased photosensitivity, but the addition of boron to the core makes the resulting grating less stable, meaning that it erases more quickly over time. One can also decrease the wavelength of the UV light below 200 nm, where the sensitivity of fiber increases, however such short-wavelength laser sources are difficult to operate. Exotic and expensive materials must be used for the optical elements that transmit laser light at such short wavelengths. The most common source for such laser light is the argon/fluorine excimer laser, whose fluorine gas is exceedingly reactive and poisonous.
The present invention includes a method and a means for stretching an optical fiber while illuminating the fiber, either transversely or by sending light through the fiber, in order to write a grating. The stretching is done in such a way that it increases photosensitivity and reduces time for writing a grating.
The fiber can be stretched by first wrapping a few turns of the fiber around a suitable mandrel, or the fiber can be damped. A separation between two portions of the fiber is increased until a desired strain (i.e., xcex941/1) is obtained. Alternatively, the fiber can be compacted by applying an inwardly directed radial force to the fiber. Alternatively, the fiber can be compacted by applying a compressive force along the axis of the fiber.
The grating can be formed in the core of the fiber or in the cladding of the fiber, depending on the composition of the fiber and how the incident light is focused onto the fiber. The light can be ultraviolet, visible light, or infrared.
The glass fiber can be stripped of all or a desired portion of a plastic coating formed around the cladding before the fiber is stretched and illuminated. Alternatively, the fiber can be stretched and illuminated with its plastic coating intact. In the latter case, the wavelength of the light must pass through the fiber coating with minimal loss, as described in U.S. Pat. No. 5,881,186. For some plastic coatings, it would be difficult to write a reasonably strong grating (R greater than 1%) through the coating of such unstretched fibers because the optical fluence required to do so would destroy the coating. Alternatively, the fiber may have a thin layer of metal coating the fiber cladding.
By stretching the fiber, the optical fluence needed to write a grating (the inverse of photosensitivity) is greatly reduced (e.g., by a factor of 20), so that the time that the fiber must be exposed to light is reduced by the same factor. By stretching the fiber, a grating can be written through the plastic coating of commercially available conventional fibers without damaging the fiber coating. Therefore this method enables gratings to be written by directing light transversely through the fiber coating in some fibers, where without the stretching the fiber coating would be destroyed before the grating could be written. The fiber core can also be illuminated by shining light along the axis of the fiber while the fiber is under stress.
As described in the preferred embodiments, the present invention thus includes a method for faster writing of gratings in optical fibers having small concentrations of Ge in their core; an efficient method for writing gratings in optical fibers without a fluorine-based excimer laser; a method for writing gratings in optical fibers without hydrogen loading the fibers; a method for writing gratings in optical fibers without compromising the long-term stability of the resulting grating; a method for writing gratings in optical fibers that enables one to write through a polymer coating of the fiber without damaging the coating; and a method for writing gratings in optical fibers that is simple and easy to implement, thereby increasing the speed and decreasing the cost of grating manufacture.
Other features and advantages will become apparent from the following description, drawings, and claims.